Incorporeal is a hard thing to pin down. By definition, it is something without matter or physical existence. According to real world physics, that means it would have to be made up of one of the fundamental forces of nature such as magnetism or gravity. Of course in a fantasy setting, you could also use the fundamental force of magic to explain it, or a spiritual force. Why physical attacks are able to damage them at all is a little difficult to grasp, but my best explanation is that it is the kinetic energy of the blow rather than any physical contact that it is doing the damage. It could also have to do with the gravity and other forces being exuded by the object you attack with. Regardless, it is not very powerful against them. You need an attack that also transcends the physical, i.e. ethereal, in order to damage them.
So, basically the answer is "Magic," which does indeed allow for a sentient smell personified if that is what people want. That's not really something I'm on board for though.